j BYZANTINE ARCHITECTURE 25
Chora between the church and the parecclesion (p. 311)5 is covered with a barrel vault evidently built without centering. The space is first narrowed by two corbelled courses of stone and, above them, by three projecting courses of brick. From this springs the vault, built from each end in strongly inclined st:;.,ruvnts. These segments meet in the middle, leaving a diamond-shaped space filled in with longitudinal courses. Like the stairs in the Pammakaristos, this passage is very narrow, some 85 cm., yet the builders thought it ii','a.'V';;;iry to corbel out five courses before venturing to throw a vault: without ^/ntvnnu-.
Near the Pantokrator is an octagonal building, now Suleiman Aga Mesjedi but ycjK-nilly regarded as a liy/aritluo library, which has on each sitle a large wall arch, strongly elliptical in form (p. 270), T\vo arches of somewhat similar form and, apparently original are found in the south end of the gynecacum of the Pantokrator (p, 2^7), These arches may have been built in this manner to economise centering. Still, in the library they are wall arches easily constructed without: centering.
Failing the examination of a larger number of buildings in Constantinople we can hardly judge of the later methods of vault and arch constructions but one point may be further noticed. The wall internally is often set: back slightly at each spring-rourse, so that: with the prnuvtlrw of the course a considerable ledge or shelf is left* On this ledge centering could easily be supported and, would have required no further framework to the f.;rounJ. Centering seems to have been used for domes, arches, vaults, and door and window openings. It was not used in small vaults. But it is difficult in inui:.Mitr any method of constructing such groined vaults us those found in the narthexes or the Piintokmtor without a very considerable amount of centering*
7V<w,"......-As a general rule tic rods or beams were used,
either of iron or wood In the latter case they were painted with leaf or fret onmmrnts, and were evidently considered as natural features* But large vaults are often found without such tics as in the n&rthcx of the Putitokrator* Many